1. Field of the Invention
This invention relates to electronic musical instruments and more particularly to a circuit for producing a chorus or ensemble effect in such an instrument.
2. The Prior Art
Electronic musical instruments such as electronic organs, electric pianos, and various special purpose electronic instruments are designed to simulate the sound of a particular musical instrument, or to simulate the sound of one of a selected group of musical instruments. In recent years, a chorus or ensemble effect has been added to some of these instruments which permits a single instrument to simulate the sound of a group of instruments such as, for example, a string ensemble or string section of an orchestra, or the brass section of an orchestra. One existing system for providing this effect employs three independent tone oscillators plus two vibrato oscillators for each note of the scale. While the ensemble effect obtainable with this circuit is excellent, the cost of this system is sufficiently high so as to severely restrict its use.
Less expensive systems employ a standard organ or other electronic musical instrument tone generator system and apply the tone signal to a selected number (normally three) of separate delay line channels. Each of these channels contains an analog delay line through which the tone signal is applied. The clock or stepping pulses for each of the delay lines are obtained from circuitry which includes a separate voltage controlled oscillator for each channel. The control voltage input to each of these oscillators is out of phase with the control voltage inputs to each of the other oscillators.
While these systems are substantially less expensive than the first system indicated above, there is a predetermined rhythmic relationship between the tone on the various channels which results in a sound which is less aesthetically similar to that of a true ensemble than that obtainable with the first system indicated above. Another problem with these systems stems from the fact that the output from each voltage controlled oscillator varies linearly with changes in the applied control voltage while the frequency intervals between tones of the musical scale are nonlinear (exponential). Since the control voltage for the oscillator for each channel is normally formed of a low frequency sine wave having a higher frequency tremolo-inducing sine wave superimposed thereon, the nonlinearity indicated above may result, among other things, in undesirable surges in the tremolo signal rather than in a uniform, smooth tremolo. Because of the components utilized, these prior art systems also suffer from possible stability problems. Their range of control is also limited, and they are still relatively complex and expensive. Finally, existing systems utilize only a single tone signal in producing the chorus effect and are thus able to produce a chorus effect having only a single voice or other tonal characteristic, such as, for example, only a string ensemble sound. These systems are not capable of reproducing the effect of an ensemble made up of a number of different musical instruments.
A need therefore exists for a simple, inexpensive system for providing a high quality chorus or ensemble effect in an electronic musical instrument. In particular, such a system should provide a substantial range of control so as to be adapted for introducing some random variation in phase shift in the various channels, providing a truer chorus effect, and should also be capable of compensating for the various nonlinearities indicated above so as to provide, among other things, a smoothly modulated audio signal output from the delay line, including a relatively smooth tremolo output. Such a system should utilize components which have good frequency stability so as to be capable of providing a uniform output under varying environmental conditions and over extended periods of time, and should use a minimum number of components so as to be as simple and inexpensive as possible. Finally, the improved system should be adapted to simultaneously utilize tone signals having different tonal characteristics so as to be capable of reproducing the sound of an ensemble made up of different musical instruments.